Aspirin is known to inhibit platelet aggregation and vascular prostaglandin production. We looked at the effects of aspirin on coronary collateral flow following coronary occlusion in the dog. Aspirin at doses of 3 and 30 mg/kg significantly increased epicardial collateral flow at 4 hours after occlusion. Flows at 5 and 30 minutes after occlusion were not significantly altered by aspirin. Endocardial collateral flow was not influenced by the presence of aspirin. Aspirin at doses of 3 and 30 mg/kg also fully inhibited platelet aggregation. Thus, aspirin, in doses totally inhibiting platelet aggregation, significantly increased epicardial collateral flow late after occlusion.